Field of the Invention
The present invention relates a wind turbine comprising:                a wind turbine tower;        a nacelle provided on the wind turbine tower;        a rotor hub rotatably mounted to the nacelle;        at least one wind turbine blades having a blade root configured to be mounted to the rotor hub and a tip end, where the wind turbine blade further comprises a pressure side and a suction side connected to each other via a leading edge and a trailing edge, and the wind turbine blade having a relative length of 1.        
The present invention further relates to a specific method of using the above described wind turbine.
Description of Related Art
During the recent years, the size and power of wind turbines has increased along with the efficiency of the wind turbines. At the same time, the wind turbine blades have increased in size and length and the aerodynamic design has also changed. The wind turbine blade has an airfoil shaped cross-sectional profile where the sizes of the airfoil shaped profiles decrease towards the tip of the wind turbine blade. The wind turbine blade is at the other end configured to be connected to a wind turbine rotor hub where this end typically has a circular cross-sectional profile and a reinforced structure.
FIG. 1 shows a sketch of an exemplary wind turbine blade 1 having a plurality of cross-sectional profiles along the length of the wind turbine blade. The wind turbine 1 has an aerodynamically shaped configuration which comprises a leading edge 2 and a trailing 3. The wind turbine 1 comprises a blade root 4 configured to be connected to a wind turbine rotor hub (not shown). The free end of the wind turbine 1 is configured as a tip end 5 closing off the aerodynamic profile. The cross-sectional profiles each form an inscribed circle defining a common straight center line 6 extending in a longitudinal direction of the wind turbine blade 1 where the center line 6 is parallel to a rotation plane defined by the wind turbine blades 1. FIG. 2 shows the different cross-sectional profiles seen from the blade root 4 towards the tip end 5. The blade root 4 has a circular shaped profile which changes into a more aerodynamically shaped profile 7 where the size of the aerodynamic profile decreases towards the tip end 5, as shown in the figure, thereby forming a transition area between the two profiles at both the suction and pressure side where the thickness of the profile decreases towards the tip end 5 in a non-linear manner, i.e., forming a concave surface seen from the center line as indicated in FIG. 7. The wind turbine 1 has a twisted longitudinal profile where the chord lines of the different cross-sectional profiles are placed in an increasing/decreasing angle relative to a reference cord line 8, e.g., of the tip end 5, so that the angle of attack is optimized along the length of the wind turbine blade 1. In this configuration, the dominant aerodynamic loading causes the wind turbine blade to bend towards the wind turbine tower. The wind turbine blade will then act as a beam with the suction side in compression and the pressure side in tension. This increases the risk of the wind turbine blade hitting the wind turbine tower and reduces the swept area of the wind turbine blades, thereby reducing the efficiency of the wind turbine. The bending of the wind turbine blades, particularly at the suction side, may lead to waves (buckles) forming in the wind turbine blades causing the blades to fail.
International Patent Application Publication WO 99/14490 A1 and corresponding U.S. Pat. No. 6,582,196 B1 disclose a wind turbine having a number of pre-bent wind turbine blades where the tip end of the wind turbine blades is bent outwards away from the wind turbine tower. The wind acting on the pressure side will press the tip end backwards so that the blade is straightened, thereby increasing the swept area. This configuration has the drawback that the thickness distribution of the blade profile along the length of the wind turbine blade results in a S-shaped main load path forming in the spar cap of the suction side at the transition area, which occurs during wind loads. This may lead to a pre-buckling of the laminate along this spar cap; thus forming an imperfection in the laminate where the deflection may increase infinitely during wind loads. This causes the compression load to be accompanied by local bending loads formed in the laminate along the S-shaped section; this causes earlier failure of the wind turbine blade.
German Utility Model DE 20120324 U1 discloses a wind turbine with three wind turbine blades each having in the span-wise direction of the blade a straight surface at the pressure side and a concave surface at the suction side. The blade forms at the transition area a relative large convex section followed by a relative large concave section which define the main load path in the suction side. This will increase the compression load at the suction side and thus the bending loads in these sections due to the wind loads acting on the wind turbine blade. This increases the risk of the laminate buckling and causing a failure.
U.S. Pat. No. 7,832,985 B2 discloses a wind turbine having a number of pre-bent wind turbine blades. The cross-sectional profiles of the wind turbine blades have a thickness distribution that decreases linearly towards the tip end for reducing the compression loads. This forms a straight surface at both the pressure and suction sides. The cross-sectional profiles are aligned according to the center line extending along the length of the wind turbine blade where a portion of the center line is placed in an acute angle relative to the center line at the blade root. This blade profile does not specify the thickness or the slenderness of the innermost section of the wind turbine blade.